1. Field of the Invention
The present invention relates to an AlN structure having anisotropy in physical properties such as a thermal conductivity, a light transmittance, and a strength and a method of manufacturing the same.
2. Description of the Related Art
Aluminum nitride (AlN) has a high strength at temperatures from a room temperature to higher temperatures, an excellent corrosion durability and a good thermal shock resistance, is not wet with a molten metal, and has a high insulation performance and a high thermal conductivity, i.e., has many good properties. An AlN ceramics having particularly good properties of such AlN can be obtained by a manufacturing method disclosed in, e.g., Published Unexamined Japanese Patent Application No. 60-127267.
An AlN ceramics obtained by the above manufacturing method has isotropy in various physical properties. In recent years, however, a demand has arisen for a practical AlN structure having anisotropy in various physical properties. In order to satisfy such a demand, Published Unexamined Japanese Patent Application No. 60-73843 discloses an AlN structure having thermal conductivity anisotropy. Since, however, this AlN structure is obtained by integrating AlN and an Al-O-N-system or Al-Si-O-N-system compound, good properties of AlN have not been satisfactorily utilized.
That is, a high thermal conductivity as an essential property of AlN is degraded. In addition, the resistance of the Al-O-N-system or Al-Si-O-N-system compound is degraded in severe environments such as high temperatures or metal vapors, and degradation progresses due to corrosiveness. Therefore, this AlN structure cannot be used under such conditions.
Published Unexamined Japanese Patent Application No. 64-89546 discloses a semiconductor device having a package portion obtained by bonding a member consisting of high-purity white AlN and a member consisting of low-purity black AlN obtained by decreasing a purity by adding a transition metal. In this structure, the two members having different purities are bonded by brazing or an active metal method. Therefore, a layer consisting of a substance except for AlN is formed between the two members. This is because the structure aims at protecting a semiconductor chip inside the package portion and therefore the bonding between the two members is formed in consideration of only ease in manufacturing and a bonding strength.
As described above, AlN is not used in all portions in conventional AlN structures. Therefore, since a portion consisting of a material except for AlN degrades the properties of the AlN structures, the good properties thereof are not satisfactorily utilized.